A role for inhibin and activin in primate oocyte maturation was investigated. The maturation and fertilization of rhesus monkey oocytes recovered from the excised ovaries of nine regularly cycling animals was compared for untreated germinal vesicle (GV) intact controls versus oocytes cultured in the presence of inhibin, activin, inhibin-activin, or in a combination with follistatin (FS). Oocyte progression to metaphase II (MII) and fertilization was monitored in oocytes cultured for 48 hrs. Activin alone stimulated GVB in a dose-dependent manner while both GVB and MII development was enhanced in the presence of inhibin + activin. The latter also accelerated the rate of maturation to MII. All treatment groups exhibited a higher incidence of GVB compared to controls. When follistatin was added, the stimulatory effect of activin or activin + inhibin was abolished. Exposure to medium containing inhibin + activin significantly increased the percentage of MII oocytes that fertilized compared to controls (68% vs 25%, respectively; P<0.01). Inhibin and activin are potent stimulators of primate oocyte maturation, producing mature oocytes in vitro that fertilize with high efficiency.